To determine whether Monterey Botryllus and Woods Hole Botryllus are the same species, comparisons were made of their morphology, biology, and colony specificity. In addition, matings were carried out to ascertain whether fertile [Monterey x Woods Hole] F1 progeny could be obtained. The morphology and biology of Botryllus colonies from Monterey and from Woods Hole are very similar, and fertile F1 progeny were obtained from interpopulation crosses. Therefore, we conclude that Monterey and Woods Hole Botryllus belong to the same species. However, slight differences were observed in the allorecognition reactions (colony specificity) of these two populations. Although there are some inconsistencies among the descriptions of Botryllus schlosseri and further extensive studies of Botryllus taxonomy are needed, our data indicate that Botryllus from Monterey and from Woods Hole may be designated contingently as B. schlosseri.